1. Field of the Invention
Exemplary embodiments of the present patent application relate to a sheet conveying device that attracts a sheet on top of a sheet stack and conveys the sheet to a subsequent device for image formation, and an image forming apparatus incorporating the sheet conveying device.
2. Discussion of the Related Art
Sheet conveying devices are provided to an image forming apparatus such as a digital copier, printer, facsimile machine, offset printing machine and the like and used to convey a sheet of a recording medium (hereinafter, “sheet”) through the image forming apparatus. Certain well-known sheet conveying devices attract the sheet electrostatically or with air suction.
A description is given of schematic configurations of related-art sheet conveying devices 101 and 201, referring to FIG. 1 through FIG. 4. For descriptive purposes, the sheet conveying devices 101 and 201 and units and components included therein are described in singular form and the reference numerals corresponding to these common units and components are denoted in parentheses only for the first appearance.
As illustrated in FIG. 1 through FIG. 4, the sheet conveying device (101, 201) includes a sheet container (111, 211), a sheet attraction and conveyance unit (112, 212), an air suction unit (113, 213), an air blowing unit (114, 214), a reference position detector (115, 215), an upstream position detector (116, 216), a driving unit (117, 217), and a tray elevator (118, 218).
The sheet container includes a sheet stacking tray (111A, 211A) on which multiple sheets (102, 202) (hereinafter, also “sheet stack”) are loaded thereon. A sheet (102A, 202A) placed on top of the sheet stack is hereinafter referred to as an uppermost sheet.
The sheet attraction and conveyance unit is disposed above the sheet container, and includes a first roller (112A, 212A) and a second roller (112B, 212B), a sheet conveyance belt (112C, 212C) wound around the two rollers, and an air suction unit (113, 213). When the air suction method is employed, multiple holes, not illustrated, are formed in the surface of the sheet conveyance belt so that the uppermost sheet can be attracted to the sheet conveyance belt by action of the air suction unit.
The sheet attraction and conveyance unit attracts sheets of the sheet stack horizontally loaded on the sheet stacking tray.
The air suction unit suctions the uppermost sheet placed on top of the sheet stack in a vertical direction to separate the sheet from the other sheets of the sheet stack, attract the uppermost sheet to the sheet conveyance belt, and convey the uppermost sheet to a predetermined position.
The air blowing unit is disposed at a downstream side of the sheet conveying device in a direction of conveyance of sheets (hereinafter, a “sheet conveyance direction D”), which is on the right side of FIG. 1 to FIG. 4. The air blowing unit blows air toward one end of the sheet stack to separate the sheets in a vicinity of the uppermost sheet from each other.
The upstream position detector is disposed at an upstream side of the sheet conveying device in the sheet conveyance direction D, which is on the left side of FIG. 1 to FIG. 4 and is not affected by the air supplied from the air blowing unit. Details of the upstream position detector will be described later. The driving unit is connected to the first roller to rotate the sheet conveyance belt. The sheet lifting driver is connected to the sheet stacking tray to move the sheet stacking tray in the vertical direction.
The difference between the sheet conveying devices 101 and 201 is in the configurations of the upstream position detectors 116 and 216.
In the sheet conveying device 101, the upstream position detector 116 includes a position sensor 116A and a top surface detection lever 116B. The sheet stack 102 moves up along with elevation of the sheet stacking tray 111A, so that the uppermost sheet 102A contacts the top portion of the detection lever 116B. This action exerts an upward force on the bottom surface detection lever 116B, thereby enabling the position of the uppermost sheet 102A to be detected.
By contrast, the upstream position detector 216 of the sheet conveying device 201 includes an angle detection sensor 216A, a top surface position, detecting lever 216B, a fulcrum 216C, and a driven contact roller 216D. The driven contact roller 216D contacts the uppermost sheet 202A at an upstream side in the sheet conveyance direction D and is rotated with the movement of the uppermost sheet 202A. The top surface position detecting lever 216B has one end supporting the driven contact roller 216D and the other end movable about the fulcrum 216C. The upstream end detector 216 detects the position of the uppermost sheet 202A based on the angle detected by the angle detection sensor 216A.
In the sheet conveying device, it is important that the uppermost sheet is located at an appropriate position to be attracted to the sheet conveyance belt and that the correct position of the uppermost sheet is detected.
To address the above-described problems, one solution involves the sheet conveying device that uses a reference position detector (115, 215) disposed at a downstream side of the sheet conveying device in the sheet conveyance direction. With this configuration, the reference position detector detects that the uppermost sheet moved up by the sheet lifting driver has reached a given position to be attracted by the sheet conveyance belt, and the position of the uppermost sheet is stored in an analog-type upstream position detector, disposed upstream from the reference position detector, as disclosed, for example, in Japanese Patent Application Publication No. 2007-045630 (JP-2007-045630-A1).
According to the method disclosed in JP-2007-045630-A1, during the sheet conveying operation in which the uppermost sheet is being conveyed toward the image forming section, the position of the uppermost sheet is not detected in a downstream area where the uppermost sheet may be affected by attraction to the sheet conveyance belt and/or rise of the leading edge thereof due to air blow, but can be controlled by a position (height) of the trailing end in an upstream area. Therefore, multiple sheet feeding error and no sheet feeding can be prevented effectively.
Further, another solution involves a detecting unit that is provided to detect the position of an uppermost sheet of a sheet stack at ends of the extreme upstream portion and the extreme downstream portion. According to detection signals of the detecting unit, the height of the sheet stack can be controlled. (For example, Japanese Patent Application Publication No. 11-322101 (JP-H11-322101-A1).)
However, in the technique proposed in JP-2007-045630-A1, as illustrated in FIG. 4, the uppermost sheet is constantly pressed downward by the upstream position detector. Therefore, when the uppermost sheet becomes attracted to the sheet conveyance belt, the uppermost sheet is constantly embossed or has convex and concave portions. Therefore, a greater suction force may be necessary depending on sheet size, sheet basis weight, and sheet type, and therefore it was likely to cause adverse affect to the size of the apparatus, the amount of power consumption, noise, etc.
Further, the technique disclosed in JP-H11-322101-A1 requires an even greater suction force than that required by the technique in JP-2007-045630-A1, which is also likely to adversely affect to the size of the apparatus, the amount of power consumption, noise, etc.